Nowadays, extra-heavy crude oil called oil sand is receiving a lot of attention due to the development of oil extractive technologies. High-viscosity crude oil such as heavy oil called “bitumen” which is contained in oil sand cannot be recovered from an oil well using an ordinary method. Therefore, an extractive technology called an in-situ recovery method, in which crude oil is recovered by injecting high-temperature vapor having a temperature higher than 300° C. into an oil sand-bearing stratum to decrease the viscosity of the crude oil and by pumping up the dissolved heavy oil component called “bitumen”, is being developed. Examples of a method of injecting high-temperature vapor into an oil sand-bearing stratum include a steam injection method. In that method, vapor heated up to a high temperature is transferred through a steam line and injected through an injection pipe.
For a steam line through which vapor is transferred to a vapor injection well, seamless steel pipes or UOE steel pipes manufactured by performing welding using a weld metal have been used from the viewpoint of the reliability of pipes. Since the electric resistance weld zone of an electric resistance welded pipe is poor in terms of reliability, electric resistance welded pipes have not been used for use applications in which a high-temperature strength property is required. Electric resistance welded pipes have been used only for the parts used in near room temperature and for which a high-temperature strength property is not required.
To solve such a problem, for example, Japanese Unexamined Patent Application Publication No. 2006-183133 describes a method of manufacturing a high strength steel pipe for a steam line having excellent toughness in a welded heat-affected zone. The technique described in Japanese Unexamined Patent Application Publication No. 2006-183133 is a technique in which a welded steel pipe is manufactured by heating a steel slab, having a chemical composition containing, by mass %, C: 0.05% to 0.09%, Si: 0.05% to 0.20%, Mn: 1.5% to 2.0%, P: 0.020% or less, S: 0.002% or less, Mo: 0.05% to 0.3%, Nb: 0.005% to 0.05%, Ti: 0.005% to 0.02%, Al: 0.01% to 0.04% and N: 0.004% to 0.006% in which the condition that Ti/N is 2.0 to 4.0 is satisfied, up to a temperature of 1000° C. to 1200° C., by hot-rolling the heated steel slab under conditions such that the cumulative rolling reduction at a temperature of 900° C. or lower is 50% or more and the rolling finishing temperature is 850° C. or lower, by cooling the hot rolled steel sheet down to a temperature of 400° C. to 550° C. by performing accelerated cooling at a cooling rate of 5° C./sec. or more, by forming the obtained steel plate into a pipe shape by performing cold forming and by welding the butt portions of the formed steel plate. According to the technique described in Japanese Unexamined Patent Application Publication No. 2006-183133, it is possible to obtain a high strength welded steel pipe for a steam line having high yield strength and high toughness in a welded heat-affected zone at a temperature of 350° C.
In addition, Japanese Patent No. 4741528 (Japanese Unexamined Patent Application Publication No. 2008-195991), describes a method of manufacturing a steel pipe for a steam transporting line with excellent high-temperature properties. The technique described in Japanese Patent No. 4741528 (Japanese Unexamined Patent Application Publication No. 2008-195991) is a method of manufacturing a high strength steel pipe for a steam transporting line with excellent high-temperature properties, the method including heating a steel slab, having a chemical composition containing, by mass %, C: 0.02% to 0.10%, Si: 0.01% to 0.50%, Mn: 0.5% to 2.0%, Nb: 0.005% to 0.050%, Ti: 0.005% to 0.050%, N: 0.001% to 0.010% and B: 0.0001% to 0.0050%, or, further containing Mo, Cr, V, Ca, REM and so forth, in which the contents of P, S and Al are limited respectively to P: 0.020% or less, S: 0.005% or less and Al: 0.04% or less, and in which the condition that Ti/N is 2.0 to 4.0 is satisfied, up to a temperature of 1000° C. to 1250° C., hot-rolling the heated slab under conditions such that the cumulative rolling reduction in a temperature range of 900° C. or lower is 50% or more and the rolling finishing temperature is 850° C. or lower, cooling the hot rolled steel sheet down to a temperature of 400° C. to 550° C. by performing accelerated cooling at a cooling rate of 5° C./sec. or more, forming the obtained high strength steel plate into a pipe shape and welding the butted portions. According to the technique described in Japanese Patent No. 4741528 (Japanese Unexamined Patent Application Publication No. 2008-195991), it is possible to manufacture a high strength steel pipe having a large diameter for a steam transportation line with excellent high-temperature properties and longtime creep properties.
However, in the techniques described by Japanese Unexamined Patent Application Publication No. 2006-183133 and Japanese Patent No. 4741528 (Japanese Unexamined Patent Application Publication No. 2008-195991), there is an inevitable welded heat-affected zone having a large grain diameter in steel pipes as a result of being heated up to a temperature just below the melting point. Since the high-temperature strength of a welded heat-affected zone decreases due to the presence of such a welded heat-affected zone, it causes concern that there may be a decrease in strength after a long period of use in an intermediate temperature range (300° C. to 400° C.). Therefore, when such steel pipes are used as steel pipes for steam line, it is necessary that a large strength safety ratio be applied, which results in a problem that a vapor temperature and inner pressure may be limited.
It could therefore be helpful to provide a high strength electric resistance welded steel pipe having a high strength of 450 MPa or more in terms of yield strength YS and excellent resistance to softening for a long period in an intermediate temperature range (300° C. to 400° C.) which can be suitably used for a steam transportation line which enables more efficient and economic extraction of heavy oil component called “bitumen” which is dissolved by injecting high-temperature vapor into an oil sand-bearing stratum without limiting a vapor temperature or inner pressure and to provide a method of manufacturing the steel pipe.